Some prior-art scanners are assembled using precision-made parts. The parts must be tooled to precise tolerances so that the distance between the medium being scanned and the multiple sensors used for scanning are kept a fixed distance apart. However, because of inconsistencies in the characteristics of lenses which are placed between the medium and the sensors in order to focus the medium's image onto the sensors, some parts of the image may be in focus while other areas are out of focus, even though the sensors, lenses, and medium are all perfectly parallel to each other.
Normally scanners are provided with a test image containing a line pattern and software which optimizes the scanning process through digital manipulation of the image. The line pattern of the test image normally contains black continuous lines that extend the length of the paper being scanned along with spaces between the lines. Both the thickness of the black lines and the thickness of the white spaces between the lines are about the same as or a little smaller than the width of the scanner's pixels. If the line pattern is slightly misaligned so that each sensor reads half a line and half a space, each sensor will read an intermediate gray, and the scanner's focus cannot be tested properly.